There has been known a phase transition type information recording medium that is based on a phenomenon of phase transition taking place in a recording layer (phase transition material layer). An optical phase transition type information recording medium is a medium where the phase transition is caused to occur by means of laser beam, namely that is used to optically recording, erasing, rewriting and reproducing information. More specifically, information is recorded on the optical phase transition type information recording medium by causing transition of state between, for example, crystal phase and amorphous phase by the heat generated through irradiation of laser beam. The recorded information is read out by detecting the difference in reflectivity between crystal phase and amorphous phase.
A rewritable optical phase transition type information recording medium allows it to erase information recorded thereon and write over other information. In this medium, the initial state of the recording layer is crystal phase in general. To record information on this medium, it is irradiated with a laser beam of high power (recording power) so as to melt the recording layer and then quickly cool it, thereby turning the portion of the recording layer that has been irradiated with the laser into amorphous phase. To erase information from this medium, it is irradiated with a laser beam having an output power (erasure power) lower than that for recording to raise the temperature of the recording layer and then gradually cool it, so as to turn the portion of the recording layer that has been irradiated with the laser into crystal phase. Accordingly, it is made possible to record new information while erasing the recorded information, by irradiating the recording layer of the rewritable optical phase transition type information recording medium with a laser beam that has been power-modulated between a high power level and a low power level.
A write-once optical phase transition type information recording medium is a medium that allows it to record information thereon only once, and does not allow it to erase or write over information. In this medium, the initial state of the recording layer is amorphous phase in general. To record information on this medium, it is irradiated with a laser beam of high power (recording power) so as to raise the temperature of the recording layer and then gradually cool it, thereby turning the portion of the recording layer that has been irradiated with the laser into crystal phase.
There is also an electrical phase transition type information recording medium that records information by applying electrical energy (for example, electric current) instead of irradiation of laser beam. Information is recorded on the electrical phase transition type information recording medium by causing transition of state between crystal phase (low resistance) and amorphous phase (high resistance) in phase transition material layer of a recording layer by the Joule heat generated by the electric current. The recorded information is read by detecting the difference in electrical resistance between crystal phase and amorphous phase.
An example of the optical phase transition type information recording medium is a 4.7 GB/DVD-RAM. Constitution of the 4.7 GB/DVD-RAM is shown in FIG. 12. The information recording medium 12 (which hereafter may also be referred to simply as medium) shown in FIG. 12 comprises an incident side dielectric layer 2, an incident side interface layer 3, a recording layer 4, a counter-incident side interface layer 5, a counter-incident side dielectric layer 6, an optional compensation layer 7 and a reflective layer 8 which are provided in this order on a substrate 1 when viewed from the side where laser beam enters. A dummy substrate 10 is adhered on the reflective layer 8 by means of an adhesive layer 9.
The incident side dielectric layer 2 and the counter-incident side dielectric layer 6 have optical functions to increase the efficiency of the recording layer 4 to absorb light by regulating the optical distance and to increase the difference in the reflectivity between crystal phase and amorphous phase so as to increase the intensity of signals. These dielectric layers also have a thermal function to thermally insulate the substrate 1, the dummy substrate 10, etc. that are vulnerable to heat, from the recording layer 4 that is heated to a high temperature during recording. (ZnS)80(SiO2)20 (mol %) that is used to form the dielectric layer in the prior art is an excellent dielectric material having transparency, high refractive index, low heat conductivity, high thermal insulation, good mechanical characteristics and high humidity resistance.
The recording layer 4 is of a fast-crystallizing material containing (Ge—Sn) Te—Sb2Te3, which is prepared by substituting a part of Ge with Sn in a GeTe—Sb2Te3 quasi-dual component phase transition material, a mixture of compounds GeTe and Sb2Te3. The recording material of this material not only has high initial rewriting performance but also has high archival characteristic (capability to reproduce recorded information after a long period of storage) and high archival overwrite characteristic (capability to erase or rewrite recorded information after a long period of storage).
The reflective layer 8 has an optical function to increase the amount of light absorbed by the recording layer 4. The reflective layer 8 also has a thermal function to quickly diffuse the heat generated in the recording layer 4 and facilitate the phase transition of the recording layer 4 into amorphous phase. The reflective layer 8 further has a function to protect the multi-layer film from the operating environment.
The incident side interface layer 3 and the counter-incident side interface layer 5 have the function to prevent material transfer from occurring between the incident side dielectric layer 2 and the recording layer 4 and between the counter-incident side dielectric layer 6 and the recording layer 4. The material transfer is the diffusion of S (sulfur) into the recording layer in the course of repetitive irradiation of the recording layer 4 with the laser beam for recording and rewriting cycles, in the case where the incident side dielectric layer 2 and the counter-incident side dielectric layer 6 are of (ZnS)80(SiO2)20(mol %). Diffusion of sulfur in the recording layer causes deterioration of overwrite cycle-ability. In order to prevent overwrite cycle-ability from deteriorating, it is preferable to use a nitride containing Ge to form the incident side interface layer 3 and the counter-incident side interface layer 5 (refer, for example, to Japanese Unexamined Patent Publication (Kokai) No. 10-275360 (FIG. 2, pp 2-6)).
Based on the findings described above, the 4.7 GB/DVD-RAM was commercialized by achieving high overwrite performance and high reliability.
In the meantime, various technologies have been investigated for the purpose of further increasing the recording capacity of the information recording medium. With regards to the optical phase transition type information recording medium, for example, such a technology has been under development that employs smaller spot of laser beam so as to record information with higher density. Specifically, it is investigated to use a blue-violet laser that has shorter wavelength than the red laser used in the prior art and use an objective lens having higher numerical aperture (NA) with decreased thickness of the substrate on the side where the laser beam enters. When information is recorded with laser beam of decreased spot size, the area irradiated by the laser beam becomes smaller so that the density of power absorbed by the recording layer increases, thus resulting in greater volume change. As a result, material transfer becomes easier to occur and the use of a material containing S such as ZnS—SiO2 in contact with the recording layer leads to deterioration of the overwrite cycle-ability.
Such an optical phase transition type information recording medium has also been developed that has two information layers each having a recording layer (may hereafter be referred to as double-layer optical phase transition type information recording medium) (refer, for example, to Japanese Unexamined Patent Publication (Kokai) No. 2000-36130 (FIG. 2, pp 2-11) and Japanese Unexamined Patent Publication (Kokai) No. 2002-144736 (FIG. 3, pp 2-14)). The double-layer optical phase transition type information recording medium may have a recording capacity twice as large as the medium shown in FIG. 1. Recording and reproduction of information on and from the two information layers are carried out by means of a laser beam that enters on one side of the medium. Accordingly, recording and reproduction of information on and from the information layer located away from the surface where the laser beam enters (hereafter referred to as the second information layer) are carried out by means of a laser beam that has transmitted through the information layer located nearer to the surface where the laser beam enters (hereafter referred to as the first information layer). Therefore, the recording layer of the first information layer is made extremely thin, so as to increase the light transmittance. However, when the recording layer becomes thinner, it is subjected to the material transfer from the adjacent layers more significantly, and therefore the use of a material containing S such as ZnS—SiO2 leads to rapid deterioration of the overwrite cycle-ability.
In the past, the present inventors formed interface layers from a nitride that contains Ge on both sides of the recording layer so as to mitigate the influence of the material transfer and prevent overwrite cycle-ability from deteriorating in the high-density recording medium and the double-layer optical phase transition type information recording medium described above, similarly to the case of 4.7 GB/DVD-RAM.
However, in the optical phase transition type information recording medium where information is recorded in high density by using the laser beam of small spot size, much heat may be generated in the recording layer as higher energy (laser power) is applied to the recording layer when recording information. As a result, when the interface layer is of a nitride containing Ge as in the prior art, the interface layer may be destroyed by the heat generated in the recording layer. Destroyed interface layer is not capable of suppressing the diffusion of S from the dielectric layer. Thus the interface layer of a nitride containing Ge has the problem that it can cause rapid deterioration of overwrite cycle-ability.
Also nitride containing Ge has high heat conductivity. As a result, increasing the thickness of the interface layer for the purpose of suppressing the diffusion of S from the dielectric layer makes it easier for heat to diffuse. Thus the interface layer of a nitride containing Ge also has the problem that it can cause deterioration of sensitivity to recording.